The overall aim of this thesis is to investigate the effectiveness of a 5-minute online mindfulness practice, and test its applications to social judgements including attribution and decision-making. The seven experiments (N = 959) presented in this thesis address an important gap in the current literature on mindfulness. Specifically: 1) the empirical test of the effectiveness of a 5-minute, single-session, online mindfulness manipulation and; 2) the impact of a brief mindfulness manipulation on social judgements. At present, the majority of mindfulness research has focused on multiple sessions of practice over a number of weeks as part of a course, usually aimed at clinical populations, and at enhancing trait mindfulness (Brown & Ryan, 2003). There is evidence that such courses can be effectively delivered online (Allexandre, Neuman, Hunter, Morledge, & Roizen, 2012; Krusche, Cyhlarova, King, & Williams, 2012; Krusche, Cyhlarova, & Williams, 2013; Morledge et al., 2013) and emerging evidence for the use of single-session mindfulness with non-clinical samples (Erisman & Roemer, 2010; Heppner et al., 2008; Hong, Lishner, & Han, 2014; Hooper, Erdogan, Keen, Lawton, & Mchugh, 2015a; Jordan, Wang, Donatoni, & Meier, 2014; Kiken & Shook, 2011; Papies, Barsalou, & Custers, 2012; Weger, Hooper, Meier, & Hopthrow, 2012) that aims to increase state mindfulness (Bishop et al., 2004; Lau et al., 2006). In addition, although mindfulness exercises are readily available online and via smartphone apps, there has yet to be an empirical investigation of the effectiveness of self-help online practices, and whether brief, single-session practices actually enhance levels of mindfulness. Based on evidence that some people prefer to complete such practices in their own surroundings (Beattie, Shaw, Kaur, & Kessler, 2009; Cavanagh et al., 2013), and that a smartphone app was preferred to an in-person and web-based mindfulness practice, it is expected that a short (5-minute) single-session, online mindfulness manipulation will effectively increase state mindfulness, measured by the Toronto Mindfulness Scale (TMS, Lau et al., 2006). Mindfulness is thought to be effective in slowing automatic responding (Jordan et al., 2014; Kiken & Shook, 2011; Papies et al., 2012) and may reduce reliance on previously learnt associations (Langer & Moldoveanu, 2000b), allowing attention to be refocused on aspects of the environment that usually go unnoticed. As such, it has the potential to reduce errors in attribution. Reliance on automatic processes in social judgements can be detrimental for social harmony. For example, the mindless use of heuristics and stereotypes in person judgement can lead to prejudice and discrimination (Abrams, 2010). Furthermore, dysfunctional group dynamics can lead to poorly made decisions (Berger & Zelditch, 1998; Larson, Foster-Fishman, & Franz, 1998; Stasser & Stewart, 1992; Stasser, Taylor, & Hanna, 1989). With this in mind, the beneficial effects that mindfulness can have on interpersonal relationships (e.g. increased empathy; Block-Lerner, Adair, Plumb, Rhatigan, & Orsillo, 2007) should also help to improve group decision-making. The core aim of this thesis is to test whether a 5-minute, single-session, online mindfulness manipulation effectively increases state mindfulness, and then apply this to social judgements. Specifically, whether the mindfulness manipulation is effective in reducing attribution errors, and improving group decision-making. It is expected that after the mindfulness manipulation, participants will be less likely to respond in an automatic way when asked to attribute another individual's behaviour, or the cause of a situation based on limited information. Moreover, this is expected to improve the social experience of individuals working in groups and therefore increase decision-making accuracy. This thesis presents seven experiments in which a 5-minute mindfulness manipulation is tested in different settings (Chapter 4), applied to two attribution errors (Chapters 5 and 6), and used before a group decision-making task (Chapter 7). A summary of the findings, and the theoretical and practical implications of the findings are presented alongside limitations and avenues for future research in the final chapter of this thesis (Chapter 8).